This invention relates to laser crystallization of large-area hydrogenated amorphous silicon alloy cladding layers in a p-i-n device structure to reduce defect density in and improve the doping efficiency of the p- and n-type layers.
There are three large-area applications of hydrogenated amorphous silicon and its alloys currently being pursued worldwide. One application is the direct photovoltaic conversion of sunlight into electricity using p-i-n device structures (radiation detection (sensors) of both optical and ionizing radiation). The second is the use of thin film transistor driver arrays. The third is the use of p-i-n arrays for light-emitting diode (LED) devices. The present invention relates to methods which lower the cost of production and increase the efficiency of thin film transistor driver arrays, radiation sensors, and LED's using amorphous silicon and its alloys.
The use of hydrogenated microcrystalline silicon (.mu.c-Si:H) in such devices is known because of its high conductivity and mobility, which can be orders of magnitude higher than hydrogenated amorphous silicon (a-Si:H), which makes .mu.c-Si:H, like a-Si:H, a promising candidate for large-area displays and sensors. On the other hand, the low thermal growth rates of .mu.c-Si:H are undesirable for efficient large-scale device fabrication. As used herein, the terms "large-scale" and "large-area", when referring to techniques and devices, are used interchangeably and refer to techniques and devices which do not require crystalline substrates. Thus, large-area electronics refers to the use of amorphous substrates such as quartz, fused silica or other inexpensive amorphous "glasses." Another disadvantage of .mu.c-Si:H is that both thermal and plasma enhanced growth of .mu.c-Si:H alloyed with Group IVa elements such as germanium and carbon, important for many sensor applications to detect certain kinds of radiation, is not completely reliable.
The above mentioned problems with .mu.c-Si:H in sensor applications have led to the increased use of a-Si:H and its Group IVa alloys in p-i-n sensor structures. A limitation to the performance of such devices is the large defect density in the n- and p-type doped alloy cladding layers. The defect density in a-Si:H and alloys increases in response to dopant incorporation during film growth as a result of defect formation reactions enabled by the disordered amorphous structure. More efficient charge collection and improved device performance might be possible by reducing this defect density without degrading the conductivity of the doped alloy cladding layers or affecting the properties (e.g., photoconductivity, resistivity, etc.) of the amorphous i-layer.
Similar problems exist when a-Si:H alloys are used in thin film transistor arrays or in light-emitting diodes (LED's). Many LED arrays are composed of III-V alloys, such as GaAs, whose direct band gaps make possible efficient radiative emission. FIGS. 1A and 1B compare direct gap (FIG. 1A) and indirect-gap (FIG. 1B) materials. However, indirect-gap materials, such as crystalline silicon, require that phonon emission (E.sub.p) or absorption accompany electronic transitions across the band gap (E.sub.g) to conserve crystal momentum, which significantly reduces the cross-section for radiative electron-hole recombination. FIGS. 1A and 1B are plots of energy (E) versus crystal momentum (k). Crystal momentum is related to the momentum (p) of an electron by the relation p=hk. Thus, the E versus k diagram is parabolic, since E.about..sup.2 /2m when k=0, where m is the rest mass of the electron. FIG. 1A shows that direct-gap materials, such as GaAs, are the most efficient radiative emitters. However, the use of III-V alloy-based LED arrays are limited to the four-inch wafer dimension, since it is difficult to obtain defect-free GaAs wafers with diameters greater than four inches.
One alternative to III-V alloys in LED structures is amorphous silicon and its alloys, whose intrinsic structural disorder removes the requirement of crystal momentum conservation and thereby leads to enhanced luminescence over that of its crystalline counterpart, as shown in FIG. 2. While the luminescence efficiency is less than in III-V alloy-based LED's, the great advantage of amorphous silicon LED technology lies in the ability to use large-area substrates. One problem hindering this development is the low doping efficiency of amorphous silicon which limits minority injection currents. This deficiency is particularly acute when wide band gap amorphous silicon alloy cladding layers are employed in the p-i-n LED device. As shown in FIG. 3, wide band gap cladding layers are desirable because they allow optical waveguiding due to different refractive indices, improved recombination efficiency in the active layer, electronic carrier confinement in the active layer, and increased LED emission energies from the active layer.
Hamakawa, et al., in their paper "Optoelectronics and Photovoltaic Application of Microcrystalline SiC", MRS Symp. Proc. (1989) disclose an electronic cyclotron resonance (ECR) plasma-enhanced CVD process which forms microcrystalline silicon alloy regions or phases imbedded in amorphous silicon alloy networks. The range of dopant concentration can be very wide. Conductivity values significantly higher than in microcrystalline silicon alloy films produced from conventional plasma-enhanced CVD processes are disclosed. An LED is disclosed, consisting of an amorphous silicon alloy p-i-n junction. To improve the injection efficiency, wide band gap p- and n-type injection materials possessing high conductivity, produced by the ECR plasma CVD process are used.
Fang, Solar Cells, 25 (1988), pages 27-29, discloses an amorphous-microcrystalline transformation in an amorphous silicon surface induced by krypton-fluorine excimer laser radiation. The process uses a single pulse of the excimer laser to produce a doped microcrystalline silicon from a doped amorphous silicon layer. Energy density of the laser used in this process is roughly 5,000 mJ/cm.sup.2.
U.S. Pat. No. 4,609,407, issued to Masao et al. discloses irradiating with a ruby laser, at an energy density ranging from 1000-3000 mJ/cm.sup.2, either in a pulse or continuous-wave mode, an amorphous silicon layer which is either doped or undoped. The amorphous layer is underlaid by a single crystalline silicon layer. By irradiating the amorphous layer with the laser, the amorphous layer is melted and recrystallized, induced by the single crystal silicon underlayer.
Other references which may be relevant include Street, R. A., Luminescence in a-Si:H, Semiconductors and Semimetals, Vol. 21, part B, pp. 197-244, Academic Press, Inc. (1984); Kruangam et al., Amorphous Silicon-Carbide Thin Film Light Emitting Diode. Optoelectronics, Vol. 1, No. 4, pp. 67-84, Jun., 1986, MITA Press; and Seibert et al., Photoluminescence in a-Si.sub.1-4 x C.sub.x :H Films. Phys. Stat. Sol. (b) 140, 311, (1987).
It would be advantageous to develop a simple and inexpensive technique for enhancing the doping efficiency of the p- and n-type a-Si:H alloy cladding layers in a p-i-n device structure, useful in thin film transistor arrays as sensors or light-emitting diodes, without affecting the i-layer photoconductivity, resistivity, radiation detection, luminescence, and other properties. It would further be advantageous to decrease the defect densities in p- and n-layers of p-i-n sensor devices without affecting the recombination properties of the i-layer.